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SN65HVD1176: Power dissipated by the driver used with a long cable

THOMAS CRAVERO
Prodigy 30 points
Part Number: SN65HVD1176

Hello, 

I would like to evaluate the dissipated power by the driver over a long cable (1km).

Using this link https://e2e.ti.com/blogs_/b/analogwire/posts/how-to-calculate-the-power-dissipation-of-high-speed-rs-485-transceivers,.

With our configurations :

VCC [V]          5,00
ICC [mA]          0,58
VOD [V]          2,50
RL [Ohm]        54,00
   
Idr [mA]        46,30

and with this formula :

PDAC = C * f * VCC * VOD

We have a Pdac around 700 mW due to the parasitic capacitor of the cable, but i think we used too big values for it.

With a standard RS485 cable, what is the parasitic capacitor per meter that we need to use ?

I saw 11pF/ft on a RS485 cable. But i don't know if it s the good value to use.

Thanks.

  • 0
    TI__Mastermind 48327 points

    Hi Thomas,

    So 11pF/ft sounds pretty reasonable for most cabling - but if you have a specific type I'd use what it is spec'd for. You could see some significant power dissipation with long cabling.

    That being said that blog post is an approximation that was used with very small capacitive loads (basically  jig capacitance + small cap) - while this is probably the quickest way to approximate - the real results could be worse because in reality 1km of transmission line is not in reality a lumped model - its a distributed one so there is potentially higher power loss depending on the transmission line effects the system may need to overcome. So I'd be aware that this result may be slightly less than what you could actually expect. 

    Best,

    Parker Dodson